This invention relates to apparatus for applying electrical pulse treatment to the heart such as for reversing tachycardias and fibrillations of the heart. More particularly, the invention relates to novel forms of electrode structure for use in such apparatus.
It is known to pace, cardiovert and defibrillate the heart using implanted electrodes and such electrodes are currently available in a variety of different constructions. Some such electrodes reside in the heart, some are affixed to the surface of the heart, and others are implanted just beneath the surface of the skin. Examples of such electrodes are disclosed in U.S. Pat. Nos. 3,942,536; 4,291,707 and 4,662,377.
Furthermore, electrodes are known which are intended for temporary residence in veins and arteries. One such example is disclosed in U.S. Pat. No. 4,660,571 which describes a lead suitable for mapping, ablation and/or pacing.
In their simplest form, most two electrode defibrillation/cardioversion lead systems can be modeled as a series of one or more resistors across the output of a signal generator. This model incorporates five series resistors which represent the two electrodes, their conductors and the tissue between the electrodes. These resistors form a voltage divider controlling both the current flowing in the circuit and the voltage drop across each component. Since the purpose of defibrillation is to stimulate tissue, the voltage drop across the conductors and electrode interfaces is wasted energy. Therefore, maximizing defibrillation efficiency is based on minimizing the electrode interface impedance (near field impedance).
The current distribution across an electrode surface is determined by ohmic (field) rather than kinetic (chemical) factors when voltages exceed approximately 30 volts. Thus, the current density around the perimeter or physical extremes of defibrillation electrodes are dramatically higher than at their center. Several consequences result. First, the center of a planar electrode is of marginal significance and can be eliminated with relatively little impact on system impedance. Second, the larger the perimeter of an electrode, the lower the near field impedance of the electrode. This decrease in what is commonly referred to as "interface impedance" increases efficiency by increasing the percentage of the electrical voltage delivered to the tissue. Finally, increasing the separation distance between adjacent active surfaces of the same polarity reduces the electrode's near field impedance by decreasing the current density between surfaces. Thus, within limits (approximately 3 cm), there is an advantage to increasing the separation distance between adjacent electrode surfaces.
It is also known that the distribution of current density with respect to the heart affects the efficiency with which a heart is defibrillated.